The so-called Short Message Service (SMS) is a point-to-point or point-to-multipoint service which enables a user to send messages to and receive text messages from other users of a communications network. The SMS attempts to deliver a message to a mobile terminal whenever the terminal is registered to the network, even when the terminal is engaged in a voice or data call. The terminal may also roam throughout the network and still be capable of sending and receiving messages. A terminal configured for SMS provides methods for the user to receive, read, write/edit, clear, send, and save messages. The connection of a standard keyboard to the terminal facilitates the generation and editing of text messages by the user.
The network stores messages in at least one Message Center (MC), and Mobile Terminated (MT) messages are sent to the terminal by the MC. Various SMS protocol layers receive the messages and check their contents. If the contents are valid, and assuming that there is room for incoming text messages in the memory of the terminal, the message is received and stored. Otherwise, the message is rejected. U.S. Pat. No. 4,644,351 (Zabarsky et al.) discloses a paging system that enables messages to be sent to a remote unit, and which has a capability for the remote unit to transmit a message-received acknowledgement signal back to a central site. The paging system of Zabarsky et al. also enables a message-presented verification to be transmitted from an addressed pager back to the central site.
Reference can be had to the TIA Interim Standards IS-136 and IS-137, and also to a document entitled "TDMA Forum, Implementation Guide: Short Message Terminals Compliant With IS-136 and IS-137" (Ed. D. Holmes, vers. 7.0, Apr. 20, 1995). These documents specify one type of SMS and the various system and terminal level protocols required to provide SMS. This latter document describes in sections 3.2.1 and 3.2.2 a Delivery Acknowledgement and a Manual Acknowledgement capability, respectively. By example, a message "Can you meet me this evening? &lt;NTC&gt;Yes&lt;NTC&gt;No", where NTC is a non-text character, is said would allow the user to select a response from "Yes" or "No", and consequently cause the terminal to send response codes 0 or 1.
Reference may also be had to IS-136.1, Rev. A, Feb. 12, 1996, Section 7 (Point-to-Point Teleservices), for a description of SMS Deliver, SMS Delivery ACK, and SMS Manual ACK message formats and protocols. When the SMS Delivery ACK is specified, the mobile station automatically transmits the acknowledgement message upon delivery (i.e., display) of a stored SMS message to the user. When the SMS Manual ACK is specified, the mobile station transmits the acknowledgement message, and the user's response, after delivery and the user's input.
Another type of SMS is known as broadcast SMS (or also as S-BCCH), which is a point-to-multipoint operation (i.e., from a network operator to a plurality of mobile stations). Reference may had to, by example, a publication entitled "S-BCCH for IS-136", TR45.3.6/95.06.13/8, Digital Cellular Systems, Jun. 12-15, 1995 (Ericsson).
A problem arises when a mobile station has received and stored a SMS message that requires an acknowledgement, either Delivery ACK or Manual ACK, and then subsequently moves outside of the cellular coverage area. In this case the mobile station is unable to respond to the SMS message. For example, the mobile station may simply present a message to the user to inform the user that the required acknowledgement cannot be sent. It is then up to the user to manually retry the acknowledgement message at a later time when a connection with a network is reestablished. For obvious reasons, this solution is less than optimum, and may result in the existence of non-acknowledged SMS messages.
Also in this case the message center may retransmit the SMS message if a message center acknowledgement timer expires without an expected acknowledgement being received. As can be appreciated, the retransmission of non-acknowledged SMS messages consumes valuable network capacity and Digital Control Channel (DCCH) bandwidth.
A similar problem can arise, when the connection with the network is not lost or if a connection to a different network is established, if the DCCH is for some reason not accepting uplink R-Data, or if the acknowledgement message exceeds a length (number of characters defined in an R-Data Message Length information element) that the serving DCCH is willing to accept. R-Data is defined in IS-136.1, Rev. A, Section 6.4.4.9, as being a relay message used to carry point-to-point teleservice messages, such as the SMS Cellular Messaging Teleservice (CMT).
A related problem is experienced for mobile station originated SMS messages when it is determined that a user-composed SMS message cannot be transmitted, either because there currently is no service, or the serving network does not support mobile station originated SMS transmissions.